1. Field of the Invention
This invention relates to a method and apparatus for heat setting fastener elements on a fastener chain comprising a pair of interlocked fastener stringers each of which consists of a tape on one longitudinal edge whereof is sewn a thermoplastic wire forming the fastener elements.
2. Description of the Prior Art
A slide fastener known in the art includes fastener elements which are sewn onto one longitudinal edge of each of a pair of tapes. The fastener elements along one edge of a tape comprise a thermoplastic wire, or monofilament, which is sewn onto the tape after being formed into a coil or zig-zag configuration. Owing to residual stress which accompanies the forming of the wire, the wire experiences subsequent deformation that causes the fastener element pitch, namely the spacing between adjacent fastener elements, to change. To avoid this phenomenon, it is common practice to heat set the fastener elements by applying heat to the molded wire after it has been sewn onto the tape. One example of an apparatus for performing such heat setting is as disclosed in the specification of U.S. Pat. No. 3,730,817 issued on May 1, 1973. In the disclosed apparatus, a fastener stringer is wound about the outer periphery of a drum, the drum having sprockets projecting from its outer periphery in such a manner that the sprocket teeth are inserted between adjacent fastener elements wound on the drum. While the sprocket teeth thus maintain the fastener elements at the proper pitch, the wire is heated and thermally set by application of ultrasonic energy.
A problem is encountered with the above-described apparatus when the formed wires are fabricated by different forming machines, in which case some difference in fastener element pitch from one stringer to another cannot be avoided, or when different sewing machines are used. Specifically, even if the wires are formed by the same machine, use of different sewing machines can result in a different element pitch when the fastener elements are sewn on the tape. Accordingly, there are often occasions where the pitch or spacing between fastener elements belonging to the two stringers of one chain is different before the fastener elements are set by heat. If heat setting is carried out under these conditions while the elements are deformed until a certain pitch is attained, not only will a perfectly uniform element pitch not be obtained owing to a difference in spring-back, but a disparity in residual stress also will be brought about, the latter depending upon a difference in the degree of plastic deformation. Consequently, when a chain comprising such interlocked fastener stringers is subsequently heated for dyeing or the like, a difference arises between the element pitch on one stringer and the element pitch on the other. The inevitable result is undesirable wrinkling of the tapes.